o discover truth, how to recognize it when discovered, how to
arrive at general laws from facts collected by {29} observation or
experiment, and how to deduce new facts from those already found to be
true. It is thus the science of sciences, and finds its application in
every branch of knowledge. The training of his power of logical
thought is, therefore, one of the things that should be constantly
aimed at by the student.

Now all thinking is concerned, first of all, with _terms_ or names for
things or qualities or conceptions of some sort. Then, it is concerned
with comparisons of things, and the discovery of their identity or
dissimilarity, as when I say "Iron is a metal" or "all metals are
elements," each of which statements is a _proposition_, the truth or
falsity of which I must be able to discover. Finally, it is concerned
in deducing new propositions from old ones, and so arriving at new
truths, as when I discover from the two propositions stated above, the
new truth that "Iron is an element."

But there are many chances for error in this process; for instance, I
might say:

"To call you an animal would be to state the truth"--to which you would
agree; and, "To call you an ass would be to call you an animal"--to
which you would also agree; from which I might conclude that, "To call
you an ass would {30} be to state the truth"--which you might have a
vague idea was not true. If you wish to be sure that this conclusion
is incorrect, you must be able to show just why it is incorrect. The
study of logic would enable you to see just where the error lies. You
must not be governed by vague ideas, or you will be intellectually at
anybody's mercy.

In the logical study of _terms_, they are classified and distinguished,
and the importance made manifest of having in mind a clear definition
of the meaning of a term before reasoning about it. Many terms are
ambiguous, as already explained, and may mean many different things, as
for instance the terms "bill," "church," "evil," "value," "social
justice." Here, then, the importance of definite ideas will be
manifest.

Pascal laid down the essentials of logical method in the statement
"Define everything and prove everything." In other words, do not
attempt to think about a term until you have defined the term and have
a clear idea what it means; and insist upon proving every statement at
which you arrive, before accepting it finally and definitely; although
for